In projection television systems it is a common practice to utilize three cathode ray tubes (CRT) of different colors, namely, red, blue and green. The images formed on each of these CRT's is projected onto a screen where the three separate images are combined to form a full color picture. Lenses used in this application can be generally characterized by a construction which includes a strong negative element in the vicinity of the CRT, a strong positive power component at some distance away from said negative element, and a weaker component furthest away from the CRT. Examples of such lenses are shown in U.S. Pat. Nos. 4,300,817; 4,348,081 and 4,526,442.
In practice, the phosphors used in CRT's do not emit strictly monochromatic light. The emission spectrum of the green CRT, in particular, has significant side bands in red and blue parts of the spectrum. If projection lenses are not corrected for color, this chromatic spread will result in a lower overall image contrast and in sometimes visible color fringing. Therefore, to achieve the image quality requirements of high end projectors, including data display, intermediate definition and high definition television (IDTV and HDTV) applications, it is desirable to provide some degree of correction for chromatic aberrations in the lens.
Examples of partially color corrected lenses are disclosed in U.S. Pat. Nos. 4,530,575; 4,733,953, 4,758,074; 4,778,264 and 4,815,831. Fully color corrected lenses for data display and high definition TV applications are shown in U.S. Pat. Nos. 4,767,199 and 4,792,218.
The requirement for partial or total color correction always complicates an optical design problem. To achieve some degree of control of chromatic aberrations a combination of elements of various dispersions and powers must be used. That, in turn, makes it more difficult to correct for monochromatic aberrations like spherical, coma, astigmatism, etc. In the end, a color corrected lens become more complex, with a larger number of elements, and is more expensive to manufacture than the lens with only monochromatic aberration correction. An increased number of elements also leads to higher losses in transmission and contrast. And, since the individual elements in these lenses are quite large in size, additional increases in weight of these lenses become significant.
Additionally, it is often desirable that lenses be capable of high performance levels over a significant range of magnifications. Ideally, in front projection application the same lens might be used over a range of magnifications from 10.times. to 60.times. without any degradation in image quality. This further complicates the optical design.
Focusing very fast f/1.0 lenses covering a wide field of view has always been difficult. It is desirable to be able to use the same lens. For example, the same front projector should be easily adaptable to various size auditoriums by simply refocusing the projection lenses without any loss in image quality, or the lens used on one model of the rear projector should be also usable on a wide variety of other models and sizes of rear projection TV sets when appropriately refocused.
The most common way of focusing projection TV lenses usually involves moving the whole lens along the optical axis. When the rear strong negative element is liquid coupled to the CRT, only the front portion of the lens is moved. This focusing technique works sufficiently well over a limited range of magnifications. Variations of spherical aberrations and astigmatism usually prevent the range to be extended substantially. When conjugates are changed significantly the lens must be partially or completely redesigned.
Accordingly, the present invention provides new and improved color corrected projection lenses for CRT's of high definition while maintaining a wide field of view and a large relative aperture. The invention also provides a CRT projection lens that is capable of maintaining a high level of image quality over a wide range of magnifications. The invention also provides a lens which achieves a good image and a high degree of stability of aberrations correction with shift of conjugates in a relatively simple configuration.